The proposed research will investigate the auditory detection, discrimination, and information processing mechanisms of fishes using a combination of psychophysical and electrophysiological methods. Recent behavioral and physiological data provide strong evidence that much of the auditory processing among the fishes is accomplished by central neural mechanisms operating on temporally coded representations of the acoustic waveform. The aim of this research is to further define the characteristics of the central mechanisms using behavioral methods, and to identify those peripheral and central patterns of neural activity which are correlated with the behavioral capacities. In this way, studies of the relatively simply organized auditory systems of the fishes will contribute new information on the neural mechanisms of temporal processing to the development of vertebrate hearing theory. Psychophysical experiments will focus on the variables affecting the detection and discrimination of stimulus envelope and temporal fine structure. These studies make us of classically conditioned respiratory responses from animals restrained in a small water tank identical to that used in the neurophysiological experiments. In this way, both behavioral and physiological measurements are made in nearly identical sound fields. Electrophysiological experiments will focus on the responses of single neurons of the 8th nerve and brain. In one set of measurements, neural discharge patterns will be analyzed in response to the same stimulus set used in the psychophysical studies in order to tentatively identify the information-bearing elements (putative neural codes). For other measurements variables such as envelope modulation depth and temperature will be manipulated so as to alter or degrade the neurally coded temporal information. These data will then be used to predict detection and discrimination behavior as a test of certain coding hypotheses.